cpython-lru_cache-weakref: Doc/tutorial/datastructures.rst annotate

annotate Doc/tutorial/datastructures.rst @ 78335:fd5166fdb978 3.2

Issue #12288: Consider '0' and '0.0' as valid initialvalue for tkinter SimpleDialog.

author	Andrew Svetlov <andrew.svetlov@gmail.com>
date	Mon, 30 Jul 2012 19:59:53 +0300
parents	3550416d83b3
children	233673503217 2fad115408e9

rev	line source
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	1 .. _tut-structures:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	2
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	3 ***************
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	4 Data Structures
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	5 ***************
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	6
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	7 This chapter describes some things you've learned about already in more detail,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	8 and adds some new things as well.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	9
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	10 .. _tut-morelists:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	11
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	12 More on Lists
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	13 =============
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	14
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	15 The list data type has some more methods. Here are all of the methods of list
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	16 objects:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	17
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	18
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	19 .. method:: list.append(x)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	20 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	21
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	22 Add an item to the end of the list; equivalent to ``a[len(a):] = [x]``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	23
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	24
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	25 .. method:: list.extend(L)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	26 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	27
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	28 Extend the list by appending all the items in the given list; equivalent to
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	29 ``a[len(a):] = L``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	30
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	31
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	32 .. method:: list.insert(i, x)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	33 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	34
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	35 Insert an item at a given position. The first argument is the index of the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	36 element before which to insert, so ``a.insert(0, x)`` inserts at the front of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	37 the list, and ``a.insert(len(a), x)`` is equivalent to ``a.append(x)``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	38
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	39
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	40 .. method:: list.remove(x)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	41 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	42
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	43 Remove the first item from the list whose value is x. It is an error if there
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	44 is no such item.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	45
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	46
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	47 .. method:: list.pop([i])
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	48 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	49
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	50 Remove the item at the given position in the list, and return it. If no index
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	51 is specified, ``a.pop()`` removes and returns the last item in the list. (The
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	52 square brackets around the i in the method signature denote that the parameter
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	53 is optional, not that you should type square brackets at that position. You
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	54 will see this notation frequently in the Python Library Reference.)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	55
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	56
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	57 .. method:: list.index(x)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	58 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	59
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	60 Return the index in the list of the first item whose value is x. It is an
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	61 error if there is no such item.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	62
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	63
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	64 .. method:: list.count(x)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	65 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	66
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	67 Return the number of times x appears in the list.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	68
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	69
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	70 .. method:: list.sort()
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	71 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	72
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	73 Sort the items of the list, in place.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	74
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	75
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	76 .. method:: list.reverse()
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	77 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	78
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	79 Reverse the elements of the list, in place.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	80
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	81 An example that uses most of the list methods::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	82
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	83 >>> a = [66.25, 333, 333, 1, 1234.5]
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	84 >>> print(a.count(333), a.count(66.25), a.count('x'))
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	85 2 1 0
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	86 >>> a.insert(2, -1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	87 >>> a.append(333)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	88 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	89 [66.25, 333, -1, 333, 1, 1234.5, 333]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	90 >>> a.index(333)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	91 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	92 >>> a.remove(333)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	93 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	94 [66.25, -1, 333, 1, 1234.5, 333]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	95 >>> a.reverse()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	96 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	97 [333, 1234.5, 1, 333, -1, 66.25]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	98 >>> a.sort()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	99 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	100 [-1, 1, 66.25, 333, 333, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	101
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	102
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	103 .. _tut-lists-as-stacks:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	104
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	105 Using Lists as Stacks
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	106 ---------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	107
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	108 .. sectionauthor:: Ka-Ping Yee <ping@lfw.org>
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	109
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	110
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	111 The list methods make it very easy to use a list as a stack, where the last
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	112 element added is the first element retrieved ("last-in, first-out"). To add an
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	113 item to the top of the stack, use :meth:`append`. To retrieve an item from the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	114 top of the stack, use :meth:`pop` without an explicit index. For example::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	115
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	116 >>> stack = [3, 4, 5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	117 >>> stack.append(6)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	118 >>> stack.append(7)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	119 >>> stack
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	120 [3, 4, 5, 6, 7]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	121 >>> stack.pop()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	122 7
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	123 >>> stack
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	124 [3, 4, 5, 6]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	125 >>> stack.pop()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	126 6
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	127 >>> stack.pop()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	128 5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	129 >>> stack
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	130 [3, 4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	131
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	132
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	133 .. _tut-lists-as-queues:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	134
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	135 Using Lists as Queues
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	136 ---------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	137
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	138 .. sectionauthor:: Ka-Ping Yee <ping@lfw.org>
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	139
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	140 It is also possible to use a list as a queue, where the first element added is
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	141 the first element retrieved ("first-in, first-out"); however, lists are not
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	142 efficient for this purpose. While appends and pops from the end of list are
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	143 fast, doing inserts or pops from the beginning of a list is slow (because all
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	144 of the other elements have to be shifted by one).
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	145
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	146 To implement a queue, use :class:`collections.deque` which was designed to
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	147 have fast appends and pops from both ends. For example::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	148
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	149 >>> from collections import deque
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	150 >>> queue = deque(["Eric", "John", "Michael"])
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	151 >>> queue.append("Terry") # Terry arrives
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	152 >>> queue.append("Graham") # Graham arrives
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	153 >>> queue.popleft() # The first to arrive now leaves
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	154 'Eric'
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	155 >>> queue.popleft() # The second to arrive now leaves
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	156 'John'
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	157 >>> queue # Remaining queue in order of arrival
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	158 deque(['Michael', 'Terry', 'Graham'])
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	159
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	160
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	161 .. _tut-listcomps:
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	162
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	163 List Comprehensions
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	164 -------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	165
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	166 List comprehensions provide a concise way to create lists.
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	167 Common applications are to make new lists where each element is the result of
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	168 some operations applied to each member of another sequence or iterable, or to
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	169 create a subsequence of those elements that satisfy a certain condition.
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	170
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	171 For example, assume we want to create a list of squares, like::
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	172
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	173 >>> squares = []
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	174 >>> for x in range(10):
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	175 ... squares.append(x**2)
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	176 ...
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	177 >>> squares
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	178 [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	179
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	180 We can obtain the same result with::
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	181
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	182 squares = [x**2 for x in range(10)]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	183
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	184 This is also equivalent to ``squares = map(lambda x: x**2, range(10))``,
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	185 but it's more concise and readable.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	186
54630 2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	187 A list comprehension consists of brackets containing an expression followed
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	188 by a :keyword:`for` clause, then zero or more :keyword:`for` or :keyword:`if`
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	189 clauses. The result will be a new list resulting from evaluating the expression
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	190 in the context of the :keyword:`for` and :keyword:`if` clauses which follow it.
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	191 For example, this listcomp combines the elements of two lists if they are not
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	192 equal::
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	193
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	194 >>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	195 [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	196
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	197 and it's equivalent to::
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	198
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	199 >>> combs = []
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	200 >>> for x in [1,2,3]:
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	201 ... for y in [3,1,4]:
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	202 ... if x != y:
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	203 ... combs.append((x, y))
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	204 ...
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	205 >>> combs
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	206 [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	207
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	208 Note how the order of the :keyword:`for` and :keyword:`if` statements is the
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	209 same in both these snippets.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	210
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	211 If the expression is a tuple (e.g. the ``(x, y)`` in the previous example),
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	212 it must be parenthesized. ::
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	213
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	214 >>> vec = [-4, -2, 0, 2, 4]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	215 >>> # create a new list with the values doubled
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	216 >>> [x*2 for x in vec]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	217 [-8, -4, 0, 4, 8]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	218 >>> # filter the list to exclude negative numbers
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	219 >>> [x for x in vec if x >= 0]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	220 [0, 2, 4]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	221 >>> # apply a function to all the elements
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	222 >>> [abs(x) for x in vec]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	223 [4, 2, 0, 2, 4]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	224 >>> # call a method on each element
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	225 >>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	226 >>> [weapon.strip() for weapon in freshfruit]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	227 ['banana', 'loganberry', 'passion fruit']
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	228 >>> # create a list of 2-tuples like (number, square)
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	229 >>> [(x, x**2) for x in range(6)]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	230 [(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	231 >>> # the tuple must be parenthesized, otherwise an error is raised
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	232 >>> [x, x**2 for x in range(6)]
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	233 File "<stdin>", line 1, in ?
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	234 [x, x**2 for x in range(6)]
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	235 ^
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	236 SyntaxError: invalid syntax
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	237 >>> # flatten a list using a listcomp with two 'for'
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	238 >>> vec = [[1,2,3], [4,5,6], [7,8,9]]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	239 >>> [num for elem in vec for num in elem]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	240 [1, 2, 3, 4, 5, 6, 7, 8, 9]
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	241
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	242 List comprehensions can contain complex expressions and nested functions::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	243
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	244 >>> from math import pi
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	245 >>> [str(round(pi, i)) for i in range(1, 6)]
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	246 ['3.1', '3.14', '3.142', '3.1416', '3.14159']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	247
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	248 Nested List Comprehensions
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	249 --------------------------
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	250
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	251 The initial expression in a list comprehension can be any arbitrary expression,
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	252 including another list comprehension.
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	253
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	254 Consider the following example of a 3x4 matrix implemented as a list of
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	255 3 lists of length 4::
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	256
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	257 >>> matrix = [
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	258 ... [1, 2, 3, 4],
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	259 ... [5, 6, 7, 8],
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	260 ... [9, 10, 11, 12],
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	261 ... ]
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	262
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	263 The following list comprehension will transpose rows and columns::
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	264
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	265 >>> [[row[i] for row in matrix] for i in range(4)]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	266 [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	267
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	268 As we saw in the previous section, the nested listcomp is evaluated in
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	269 the context of the :keyword:`for` that follows it, so this example is
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	270 equivalent to::
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	271
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	272 >>> transposed = []
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	273 >>> for i in range(4):
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	274 ... transposed.append([row[i] for row in matrix])
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	275 ...
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	276 >>> transposed
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	277 [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	278
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	279 which, in turn, is the same as::
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	280
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	281 >>> transposed = []
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	282 >>> for i in range(4):
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	283 ... # the following 3 lines implement the nested listcomp
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	284 ... transposed_row = []
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	285 ... for row in matrix:
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	286 ... transposed_row.append(row[i])
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	287 ... transposed.append(transposed_row)
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	288 ...
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	289 >>> transposed
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	290 [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	291
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	292 In the real world, you should prefer built-in functions to complex flow statements.
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	293 The :func:`zip` function would do a great job for this use case::
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	294
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	295 >>> zip(*matrix)
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	296 [(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)]
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	297
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	298 See :ref:`tut-unpacking-arguments` for details on the asterisk in this line.
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	299
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	300 .. _tut-del:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	301
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	302 The :keyword:`del` statement
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	303 ============================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	304
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	305 There is a way to remove an item from a list given its index instead of its
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	306 value: the :keyword:`del` statement. This differs from the :meth:`pop` method
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	307 which returns a value. The :keyword:`del` statement can also be used to remove
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	308 slices from a list or clear the entire list (which we did earlier by assignment
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	309 of an empty list to the slice). For example::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	310
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	311 >>> a = [-1, 1, 66.25, 333, 333, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	312 >>> del a[0]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	313 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	314 [1, 66.25, 333, 333, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	315 >>> del a[2:4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	316 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	317 [1, 66.25, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	318 >>> del a[:]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	319 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	320 []
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	321
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	322 :keyword:`del` can also be used to delete entire variables::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	323
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	324 >>> del a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	325
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	326 Referencing the name ``a`` hereafter is an error (at least until another value
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	327 is assigned to it). We'll find other uses for :keyword:`del` later.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	328
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	329
49180 c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48665 diff changeset	330 .. _tut-tuples:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	331
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	332 Tuples and Sequences
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	333 ====================
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	334
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	335 We saw that lists and strings have many common properties, such as indexing and
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	336 slicing operations. They are two examples of sequence data types (see
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	337 :ref:`typesseq`). Since Python is an evolving language, other sequence data
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	338 types may be added. There is also another standard sequence data type: the
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	339 tuple.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	340
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	341 A tuple consists of a number of values separated by commas, for instance::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	342
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	343 >>> t = 12345, 54321, 'hello!'
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	344 >>> t[0]
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	345 12345
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	346 >>> t
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	347 (12345, 54321, 'hello!')
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	348 >>> # Tuples may be nested:
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	349 ... u = t, (1, 2, 3, 4, 5)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	350 >>> u
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	351 ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
77496 3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	352 >>> # Tuples are immutable:
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	353 ... t[0] = 88888
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	354 Traceback (most recent call last):
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	355 File "<stdin>", line 1, in <module>
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	356 TypeError: 'tuple' object does not support item assignment
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	357 >>> # but they can contain mutable objects:
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	358 ... v = ([1, 2, 3], [3, 2, 1])
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	359 >>> v
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	360 ([1, 2, 3], [3, 2, 1])
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	361
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	362
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	363 As you see, on output tuples are always enclosed in parentheses, so that nested
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	364 tuples are interpreted correctly; they may be input with or without surrounding
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	365 parentheses, although often parentheses are necessary anyway (if the tuple is
77496 3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	366 part of a larger expression). It is not possible to assign to the individual
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	367 items of a tuple, however it is possible to create tuples which contain mutable
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	368 objects, such as lists.
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	369
77496 3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	370 Though tuples may seem similar to lists, they are often used in different
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	371 situations and for different purposes.
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	372 Tuples are :term:`immutable`, and usually contain an heterogeneous sequence of
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	373 elements that are accessed via unpacking (see later in this section) or indexing
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	374 (or even by attribute in the case of :func:`namedtuples <collections.namedtuple>`).
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	375 Lists are :term:`mutable`, and their elements are usually homogeneous and are
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	376 accessed by iterating over the list.
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	377
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	378 A special problem is the construction of tuples containing 0 or 1 items: the
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	379 syntax has some extra quirks to accommodate these. Empty tuples are constructed
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	380 by an empty pair of parentheses; a tuple with one item is constructed by
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	381 following a value with a comma (it is not sufficient to enclose a single value
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	382 in parentheses). Ugly, but effective. For example::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	383
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	384 >>> empty = ()
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	385 >>> singleton = 'hello', # <-- note trailing comma
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	386 >>> len(empty)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	387 0
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	388 >>> len(singleton)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	389 1
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	390 >>> singleton
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	391 ('hello',)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	392
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	393 The statement ``t = 12345, 54321, 'hello!'`` is an example of tuple packing:
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	394 the values ``12345``, ``54321`` and ``'hello!'`` are packed together in a tuple.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	395 The reverse operation is also possible::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	396
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	397 >>> x, y, z = t
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	398
52892 792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 51776 diff changeset	399 This is called, appropriately enough, sequence unpacking and works for any
54630 2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	400 sequence on the right-hand side. Sequence unpacking requires that there are as
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	401 many variables on the left side of the equals sign as there are elements in the
52892 792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 51776 diff changeset	402 sequence. Note that multiple assignment is really just a combination of tuple
792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 51776 diff changeset	403 packing and sequence unpacking.
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	404
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	405
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	406 .. _tut-sets:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	407
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	408 Sets
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	409 ====
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	410
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	411 Python also includes a data type for sets. A set is an unordered collection
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	412 with no duplicate elements. Basic uses include membership testing and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	413 eliminating duplicate entries. Set objects also support mathematical operations
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	414 like union, intersection, difference, and symmetric difference.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	415
65119 f00440af9413 Merged revisions 84249,84264,84326-84327,84407,84476,84480-84482,84484,84530-84531,84553,84619,84684,84915-84916 via svnmerge from Georg Brandl <georg@python.org> parents: 63813 diff changeset	416 Curly braces or the :func:`set` function can be used to create sets. Note: To
54632 255db70bb316 Fix markup. Georg Brandl <georg@python.org> parents: 54630 diff changeset	417 create an empty set you have to use ``set()``, not ``{}``; the latter creates an
255db70bb316 Fix markup. Georg Brandl <georg@python.org> parents: 54630 diff changeset	418 empty dictionary, a data structure that we discuss in the next section.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	419
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	420 Here is a brief demonstration::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	421
63813 28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	422 >>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	423 >>> print(basket) # show that duplicates have been removed
66478 7281937b2d65 Merged revisions 86134,86315-86316,86390,86424-86425,86428,86550,86561-86562,86564-86565,86705,86708,86713 via svnmerge from Georg Brandl <georg@python.org> parents: 65119 diff changeset	424 {'orange', 'banana', 'pear', 'apple'}
63813 28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	425 >>> 'orange' in basket # fast membership testing
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	426 True
63813 28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	427 >>> 'crabgrass' in basket
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	428 False
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	429
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	430 >>> # Demonstrate set operations on unique letters from two words
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	431 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	432 >>> a = set('abracadabra')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	433 >>> b = set('alacazam')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	434 >>> a # unique letters in a
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	435 {'a', 'r', 'b', 'c', 'd'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	436 >>> a - b # letters in a but not in b
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	437 {'r', 'd', 'b'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	438 >>> a \| b # letters in either a or b
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	439 {'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	440 >>> a & b # letters in both a and b
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	441 {'a', 'c'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	442 >>> a ^ b # letters in a or b but not both
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	443 {'r', 'd', 'b', 'm', 'z', 'l'}
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	444
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	445 Like :ref:`for lists <tut-listcomps>`, there is a set comprehension syntax::
45084 38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	446
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	447 >>> a = {x for x in 'abracadabra' if x not in 'abc'}
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	448 >>> a
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	449 {'r', 'd'}
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	450
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	451
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	452
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	453 .. _tut-dictionaries:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	454
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	455 Dictionaries
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	456 ============
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	457
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	458 Another useful data type built into Python is the dictionary (see
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	459 :ref:`typesmapping`). Dictionaries are sometimes found in other languages as
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	460 "associative memories" or "associative arrays". Unlike sequences, which are
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	461 indexed by a range of numbers, dictionaries are indexed by keys, which can be
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	462 any immutable type; strings and numbers can always be keys. Tuples can be used
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	463 as keys if they contain only strings, numbers, or tuples; if a tuple contains
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	464 any mutable object either directly or indirectly, it cannot be used as a key.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	465 You can't use lists as keys, since lists can be modified in place using index
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	466 assignments, slice assignments, or methods like :meth:`append` and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	467 :meth:`extend`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	468
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	469 It is best to think of a dictionary as an unordered set of key: value pairs,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	470 with the requirement that the keys are unique (within one dictionary). A pair of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	471 braces creates an empty dictionary: ``{}``. Placing a comma-separated list of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	472 key:value pairs within the braces adds initial key:value pairs to the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	473 dictionary; this is also the way dictionaries are written on output.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	474
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	475 The main operations on a dictionary are storing a value with some key and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	476 extracting the value given the key. It is also possible to delete a key:value
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	477 pair with ``del``. If you store using a key that is already in use, the old
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	478 value associated with that key is forgotten. It is an error to extract a value
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	479 using a non-existent key.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	480
50176 ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	481 Performing ``list(d.keys())`` on a dictionary returns a list of all the keys
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	482 used in the dictionary, in arbitrary order (if you want it sorted, just use
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	483 ``sorted(d.keys())`` instead). [1]_ To check whether a single key is in the
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	484 dictionary, use the :keyword:`in` keyword.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	485
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	486 Here is a small example using a dictionary::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	487
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	488 >>> tel = {'jack': 4098, 'sape': 4139}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	489 >>> tel['guido'] = 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	490 >>> tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	491 {'sape': 4139, 'guido': 4127, 'jack': 4098}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	492 >>> tel['jack']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	493 4098
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	494 >>> del tel['sape']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	495 >>> tel['irv'] = 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	496 >>> tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	497 {'guido': 4127, 'irv': 4127, 'jack': 4098}
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	498 >>> list(tel.keys())
50176 ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	499 ['irv', 'guido', 'jack']
ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	500 >>> sorted(tel.keys())
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	501 ['guido', 'irv', 'jack']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	502 >>> 'guido' in tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	503 True
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	504 >>> 'jack' not in tel
7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	505 False
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	506
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	507 The :func:`dict` constructor builds dictionaries directly from sequences of
54773 f0195daadffb The key-value pairs can be lists or tuples or any iterable. Raymond Hettinger <python@rcn.com> parents: 54769 diff changeset	508 key-value pairs::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	509
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	510 >>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	511 {'sape': 4139, 'jack': 4098, 'guido': 4127}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	512
45084 38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	513 In addition, dict comprehensions can be used to create dictionaries from
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	514 arbitrary key and value expressions::
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	515
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	516 >>> {x: x**2 for x in (2, 4, 6)}
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	517 {2: 4, 4: 16, 6: 36}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	518
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	519 When the keys are simple strings, it is sometimes easier to specify pairs using
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	520 keyword arguments::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	521
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	522 >>> dict(sape=4139, guido=4127, jack=4098)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	523 {'sape': 4139, 'jack': 4098, 'guido': 4127}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	524
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	525
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	526 .. _tut-loopidioms:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	527
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	528 Looping Techniques
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	529 ==================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	530
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	531 When looping through dictionaries, the key and corresponding value can be
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	532 retrieved at the same time using the :meth:`items` method. ::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	533
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	534 >>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	535 >>> for k, v in knights.items():
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	536 ... print(k, v)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	537 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	538 gallahad the pure
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	539 robin the brave
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	540
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	541 When looping through a sequence, the position index and corresponding value can
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	542 be retrieved at the same time using the :func:`enumerate` function. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	543
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	544 >>> for i, v in enumerate(['tic', 'tac', 'toe']):
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	545 ... print(i, v)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	546 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	547 0 tic
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	548 1 tac
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	549 2 toe
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	550
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	551 To loop over two or more sequences at the same time, the entries can be paired
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	552 with the :func:`zip` function. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	553
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	554 >>> questions = ['name', 'quest', 'favorite color']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	555 >>> answers = ['lancelot', 'the holy grail', 'blue']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	556 >>> for q, a in zip(questions, answers):
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 46978 diff changeset	557 ... print('What is your {0}? It is {1}.'.format(q, a))
50517 83efc2467faf Fix a few remaining problems found by rstlint. Georg Brandl <georg@python.org> parents: 50515 diff changeset	558 ...
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	559 What is your name? It is lancelot.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	560 What is your quest? It is the holy grail.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	561 What is your favorite color? It is blue.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	562
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	563 To loop over a sequence in reverse, first specify the sequence in a forward
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	564 direction and then call the :func:`reversed` function. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	565
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	566 >>> for i in reversed(range(1, 10, 2)):
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	567 ... print(i)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	568 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	569 9
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	570 7
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	571 5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	572 3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	573 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	574
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	575 To loop over a sequence in sorted order, use the :func:`sorted` function which
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	576 returns a new sorted list while leaving the source unaltered. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	577
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	578 >>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	579 >>> for f in sorted(set(basket)):
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	580 ... print(f)
50517 83efc2467faf Fix a few remaining problems found by rstlint. Georg Brandl <georg@python.org> parents: 50515 diff changeset	581 ...
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	582 apple
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	583 banana
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	584 orange
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	585 pear
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	586
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	587
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	588 .. _tut-conditions:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	589
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	590 More on Conditions
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	591 ==================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	592
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	593 The conditions used in ``while`` and ``if`` statements can contain any
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	594 operators, not just comparisons.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	595
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	596 The comparison operators ``in`` and ``not in`` check whether a value occurs
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	597 (does not occur) in a sequence. The operators ``is`` and ``is not`` compare
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	598 whether two objects are really the same object; this only matters for mutable
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	599 objects like lists. All comparison operators have the same priority, which is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	600 lower than that of all numerical operators.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	601
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	602 Comparisons can be chained. For example, ``a < b == c`` tests whether ``a`` is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	603 less than ``b`` and moreover ``b`` equals ``c``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	604
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	605 Comparisons may be combined using the Boolean operators ``and`` and ``or``, and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	606 the outcome of a comparison (or of any other Boolean expression) may be negated
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	607 with ``not``. These have lower priorities than comparison operators; between
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	608 them, ``not`` has the highest priority and ``or`` the lowest, so that ``A and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	609 not B or C`` is equivalent to ``(A and (not B)) or C``. As always, parentheses
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	610 can be used to express the desired composition.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	611
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	612 The Boolean operators ``and`` and ``or`` are so-called short-circuit
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	613 operators: their arguments are evaluated from left to right, and evaluation
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	614 stops as soon as the outcome is determined. For example, if ``A`` and ``C`` are
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	615 true but ``B`` is false, ``A and B and C`` does not evaluate the expression
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	616 ``C``. When used as a general value and not as a Boolean, the return value of a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	617 short-circuit operator is the last evaluated argument.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	618
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	619 It is possible to assign the result of a comparison or other Boolean expression
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	620 to a variable. For example, ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	621
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	622 >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	623 >>> non_null = string1 or string2 or string3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	624 >>> non_null
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	625 'Trondheim'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	626
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	627 Note that in Python, unlike C, assignment cannot occur inside expressions. C
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	628 programmers may grumble about this, but it avoids a common class of problems
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	629 encountered in C programs: typing ``=`` in an expression when ``==`` was
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	630 intended.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	631
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	632
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	633 .. _tut-comparing:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	634
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	635 Comparing Sequences and Other Types
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	636 ===================================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	637
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	638 Sequence objects may be compared to other objects with the same sequence type.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	639 The comparison uses lexicographical ordering: first the first two items are
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	640 compared, and if they differ this determines the outcome of the comparison; if
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	641 they are equal, the next two items are compared, and so on, until either
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	642 sequence is exhausted. If two items to be compared are themselves sequences of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	643 the same type, the lexicographical comparison is carried out recursively. If
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	644 all items of two sequences compare equal, the sequences are considered equal.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	645 If one sequence is an initial sub-sequence of the other, the shorter sequence is
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	646 the smaller (lesser) one. Lexicographical ordering for strings uses the Unicode
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	647 codepoint number to order individual characters. Some examples of comparisons
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	648 between sequences of the same type::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	649
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	650 (1, 2, 3) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	651 [1, 2, 3] < [1, 2, 4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	652 'ABC' < 'C' < 'Pascal' < 'Python'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	653 (1, 2, 3, 4) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	654 (1, 2) < (1, 2, -1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	655 (1, 2, 3) == (1.0, 2.0, 3.0)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	656 (1, 2, ('aa', 'ab')) < (1, 2, ('abc', 'a'), 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	657
43497 6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	658 Note that comparing objects of different types with ``<`` or ``>`` is legal
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	659 provided that the objects have appropriate comparison methods. For example,
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	660 mixed numeric types are compared according to their numeric value, so 0 equals
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	661 0.0, etc. Otherwise, rather than providing an arbitrary ordering, the
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	662 interpreter will raise a :exc:`TypeError` exception.
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	663
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	664
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	665 .. rubric:: Footnotes
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	666
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	667 .. [1] Calling ``d.keys()`` will return a :dfn:`dictionary view` object. It
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	668 supports operations like membership test and iteration, but its contents
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	669 are not independent of the original dictionary -- it is only a view.

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/datastructures.rst @ 78335:fd5166fdb978 3.2